Implement lambda functions as a possible argument in Insert().OptArgs("conflict",...)#154

**Context** I want to do an "upsert" that when the entry already exists, updates an argument. However, that argument is a List, and a normal OptArg("conflict","update") replaces the old array with the new one, I want the resulting array to be the concatenation of both arrays (without duplicates). Is there any support for lambda functions in Insert.OptArg("conflict") in C#? In the Java documentation there is something like this but I can't seem to implement it in C#.  **Alternatives** I can do this in two requests, however, I want this request to be atomical and can't find any other alternatives. **Has the feature been requested before?** Not that I am aware of. **If the feature request is approved, would you be willing to submit a PR?** No